1. Field of the Invention
The present invention relates to an external-cavity tunable wavelength light source using a semiconductor laser and, more particularly, to a tunable wavelength light source capable of tuning the oscillation wavelength of a laser beam with a high reproducibility and a high resolution.
The present invention also relates to an external-cavity tunable wavelength light source using a semiconductor laser and, more particularly, to a tunable wavelength light source which uses a semiconductor laser having a phase adjustment area to achieve size reduction of the apparatus.
2. Description of the Related Art
A light source for selecting a wavelength by an external diffraction grating is conventionally known as a typical external-cavity tunable wavelength light source using a semiconductor laser. FIG. 8 is a block diagram showing the arrangement of this prior art.
Referring to FIG. 8, a light beam emitted from the AR-coated (AR: Anti-Reflection) end face of a semiconductor laser (LD) 1 is collimated by a lens 2 and incident on a diffraction grating 3. The light beam is spectrally dispersed, and only a light component having a specific wavelength returns to the LD 1 (to be described later). With this operation, a cavity (external cavity) is formed between an end face of the LD 1, which is not AR-coated, and the diffraction grating 3, and laser oscillation is performed at a wavelength determined by a cavity length L. The output laser beam is emitted from the end face of the LD 1, which is not AR-coated.
The principle of laser oscillation will be described below.
The diffraction angle of a light beam incident on the diffraction grating 3 changes in accordance with its wavelength. More specifically, a wavelength .lambda. for obtaining an exit angle .beta. satisfies the following relation: EQU m.lambda.=d(sin.theta.+sin.beta.), (m is a diffracted order=0, .+-.1,.+-.2, . . .) (1)
where d is the grating constant of the diffraction grating 3, and .theta. is the incident angle to the diffraction grating 3, as shown in FIG. 9.
From the light beam incident on the diffraction grating 3, a light component having a wavelength satisfying .theta.=.beta. in accordance with equation (1) returns to the LD 1, thereby forming an external cavity (cavity length L). The wavelength oscillated at this time is determined by the gain spectrum (A) of the LD 1, the wavelength characteristics (B) of a cavity loss which is mainly determined by the characteristics of the diffraction grating 3, and an external cavity longitudinal mode (C) determined by optical phase conditions, as shown in FIG. 10. More specifically, an external cavity longitudinal mode for maximizing a value obtained by subtracting the loss (B) from the gain (A) oscillates. In FIG. 10, oscillation longitudinal modes are external cavity longitudinal modes (D) and (E).
The external cavity longitudinal mode is a condition for forming a standing wave upon reciprocation of a light beam in the cavity, which is given by the following equation: EQU n.lambda.=2L (n is a natural number, and L is the above-described cavity length) (2)
An external cavity longitudinal mode spacing .DELTA..lambda. at this time is represented as follows: EQU .DELTA..lambda.=.lambda..sup.2 /2L (3)
The wavelength characteristics (B) of the cavity loss can be changed, as indicated by a dotted line in FIG. 10, by changing the incident angle .theta. to the diffraction grating 3 in FIG. 8. Additionally, the wavelength of the external cavity longitudinal mode (C) can be changed by moving the diffraction grating 3 along the moving direction shown in FIG. 8 (in other words, by changing the cavity length L).
Therefore, in the prior art shown in FIG. 8, a laser beam having a designated wavelength .lambda.s can be output from the LD 1 in the following manner.
(1) A rotating shaft 4 of the diffraction grating 3 is rotated to equalize the wavelength of the laser beam incident from the diffraction grating 3 onto the LD 1 with the designated wavelength .lambda.s.
That is, the incident angle .theta. is changed to satisfy equation (1).
(2) The rotating shaft 4 of the diffraction grating 3 is moved along a guide groove 5 to adjust the cavity length L such that equation (2) is satisfied.
The adjustment (2) must be performed once when a wavelength is to be oscillated at the external cavity longitudinal mode spacing .DELTA..lambda. shown in FIG. 10. However, when a wavelength is to be oscillated within the external cavity longitudinal mode spacing .DELTA..lambda., the adjustment must be performed whenever the wavelength changes.
The tunable wavelength light source apparatus with the arrangement as shown in FIG. 8 poses the following problems.
To adjust the cavity length L (in other words, to oscillate a waveform within the external-cavity longitudinal mode spacing .DELTA..lambda.) as in the adjustment (2), the rotating shaft 4 of the diffraction grating 3 must be moved along the guide groove 5. For this purpose, an electrically driven actuator, a piezoelectric element, or the like is used, which mechanically changes the cavity length L and poses the following problems.
(a) Use of Actuator
(a1) The reproducibility of the oscillation wavelength tends to be degraded because of backlash of the actuator.
(a2) It is difficult to precisely set the resolution of the oscillation wavelength because the actuator has difficulty in finely setting a length.
The reproducibility and resolution of the oscillation wavelength will be exemplified as detailed values.
A change in wavelength .DELTA..lambda.' when the cavity length L is changed by only .DELTA.L is represented as follows: EQU .DELTA..lambda.'=.lambda..times.(.DELTA.L/L) (4)
where .lambda. is the initial wavelength.
When the cavity length L is set to 30 mm, the initial wavelength .lambda. is set to 1.55 .mu.m which is often used in a communication band, and a highly precise compact actuator having normal performance such as a length reproducibility of 1 .mu.m and a length resolution of 20 nm is used as an actuator, the reproducibility and resolution of the oscillation wavelength are obtained in accordance with equation (4).
The reproducibility of the oscillation wavelength is represented as follows : EQU .DELTA..lambda.'=(1.55.times.10.sup.-6).times.{(1.times.10.sup.-6)/(30.time s.10.sup.-3)}.noteq.50 pm (5)
The resolution of the oscillation wavelength is represented as follows: EQU .DELTA..lambda.'=(1.55.times.10.sup.-6).times.{(20.times.10.sup.-9)/(30.tim es.10.sup.-3)}.noteq.1 pm (6)
(b) Use of Piezoelectric Element
The reproducibility of the oscillation wavelength is degraded because of the hysteresis of the piezoelectric element (free to extend upon application of a voltage). A practical value will be exemplified below.
When a normal piezoelectric element having a hysteresis of 10% with respect to the variable width of the length is used, and the variable width of the length is set to 2 .mu.m, the length reproducibility is obtained as follows: EQU (2.times.10.sup.-6).times.0.1=0.2 .mu.m (7)
When L=30 mm, and .lambda.=1.55 .mu.m, the reproducibility of the oscillation length can be calculated from the above result and equation (4) as follows: EQU .DELTA..lambda.'=(1.55.times.10.sup.-6).times.{(0.2.times.10.sup.-6)/(30.ti mes.10.sup.-3)}.noteq.10 pm (8)
The practical values of the reproducibility and resolution of the oscillation wavelength in cases (a) and (b) are poorer than those of the present invention (to be described later) by about 10 to 50 times.
In addition, in the tunable wavelength light source with the above arrangement, a driving component (not shown in FIG. 8) such as an electrically driven actuator or a piezoelectric element is attached to the diffraction grating 3 to move the rotating shaft 4 of the diffraction grating 3 along the guide groove 5, thereby adjusting the cavity length L, as described in (2) (in other words, oscillating a wavelength within the external-cavity longitudinal mode spacing .DELTA..lambda.).
As a result, the guide groove 5, the driving component, and the like are needed to adjust the cavity length L. An increase in number of components results in a bulky apparatus and a complex driving method. The increase in number of components and the mechanical nature of the driving component adversely affect the reliability.